A hand held scanner for an ultrasonic imaging system including a transducer is disclosed in U.S. Pat. No. 4,625,557. The scanner head includes a position encoder which provides distance increment signals as the scanner head is moved over a surface being analyzed. The position encoder is driven by a roller on the scanner head as the head is moved by hand over the surface. The position encoder drives the ultrasonic transceiver and display circuitry. The transceiver creates the pulse for the transducer. The transducer is thus pulsed from a distance increment signal. The scanner head is relatively small and may move in opposite directions on rollers in any direction.
The rollers include facing conical end sections which allow the curvature of a pipe or other surface to nest between such surfaces. The configuration of the rollers also allows the transducer to be nested between the rollers and the rollers positioned more closely together than would otherwise be the case. While the spacing of the rollers desirably enables the head to be quite small, approaching something on the order of an oversize stick of deodorant, it none-the-less makes the scanner head somewhat unstable. Even so, the scanner head may be randomly moved over a surface to generate an image showing thickness or defects in the structure. Also, while manipulating the scanner head, the operator may be watching the display and unattentive to the precise position of the scanner head and the location of the transducer on the surface being analyzed.
For a curved surface, the transducer should normally be aligned with the radius of curvature. For a pipe section the transducer should preferably always point to the center of the pipe. On planar or flat surfaces, the axis of the transducer should remain normal to the surface. With a small somewhat unstable head, care must be taken to keep the transducer in the proper position. Failure to keep the transducer in proper position may result in loss of couplant at the transducer-surface interface and the creation of air gaps. An angle variation will, in any event, affect the results obtained making them less reliable, and harder or impossible to repeat at a later date.
For many structures, proper repeatability is an important part of any ongoing analysis of corrosion or the development of possible defects. Many large structures such as process equipment or boilers are periodically taken out of service for maintenance and testing. For example, large boilers are routinely taken out of service after so many months of continuous operation. The periods of continuous operation may run a year or longer.
During such downtime, the boiler tubes, for example, may be ultrasonically tested for wall thickness or other defects. The results or images obtained are compared with the test results obtained the last time the structure was out of service. Accordingly, progressive changes can be monitored over the life of the equipment. If a meaningful comparison can not be made, then such a program has limited reliability. In order to have reliability, the testing should be done accurately and in a certain order at a known location, and then precisely repeated sometimes many months or even years later.
Accordingly, there is a need for a scanner having the advantages of a hand held scanner yet which has somewhat greater stability, and which can take measurements, reliably and repeatably at the same locations after lapses of substantial periods.